The day Dr. Horrible invented an animal-changing ray should have been the day of his greatest triumph over his nemesis, Captain Hammer.

He'd made careful, meticulous plans. He'd tested every component involved, twice. He'd used the completed ray on seventeen species of insect, three types of small rodent, the neighbour's dog, and finally, on a complaining Moist.

It worked. That was the problem.

He'd baited the trap perfectly, using a seemingly-careless blog post full of perfectly-gauged mad laughter about how nobody could stop him, exactly how Captain Hammer couldn't stop him, and detailed information about where he was going to be at the aformentioned unstoppable time.

He'd turned up five minutes late as he'd planned, found Captain Hammer standing in exactly the right place looking heroic and confused, and pulled the trigger.

His plan was perfect. He'd turn Captain Hammer into something small and helpless, mock him, then maybe give him to an overcrowded pro-euthanasia pet shelter.

Except...

Kitten. His ray had turned Captain Hammer into a tiny, fuzzy kitten, and wasn't that just cosmically unfair? He couldn't kill a kitten, even by proxy.

Dr. Horrible sighed, and lowered the ray. Even as he picked the small, yowling animal up by the scruff of it's neck, he could feel the mocking he'd get from the Evil League of Evil if they found out about this.

There was only one thing for it. He'd have to change Captain Hammer back. He already knew how to, because he was going to need Moist human again and the neighbours would notice their dog suddenly turning into a gecko, but he hadn't gotten around to building the appropriate parts yet.

Half an hour and five reinforced cat-baskets later, back in his lab, Billy concluded that he should have made the reversal ray at the same time. Turning nemesis' into kittens, it turned out, didn't get rid of the superstrength. His curtains were never going to be the same again.